Both automotive and non-automotive lighting applications exist that utilize a longitudinally-extending light source to provide area or feature lighting. Various known lighting methods utilize neon gas, fluorescent gas, or incandescent lighting.
In non-automotive applications, a wide variety of elongated light sources have been proposed. For example, a basic lighting apparatus of this type is described in U.S. Pat. No. 2,135,480 issued Nov. 8, 1938 to Birdseye. This patent discloses a reflecting glow lamp (a gas discharge lamp) which can have a nonsymmetrical cross-section that includes a trough-shaped portion. The trough-shaped portion has an exterior coating of metallic silver and a flattened transmitting face that may be clear or frosted. A possible configuration of the trough-shaped portion is substantially parabolic so that the lamp emits a large portion of its light in a concentrated beam of substantially parallel rays. The gaseous discharge lamps may contain neon, argon, carbon dioxide, or any other gaseous medium suitable for the maintenance of a gaseous discharge between suitable electrodes.
Apart from elongated gas discharge and fluorescent light sources such as that disclosed by Birdseye, elongated light guides are known that laterally emit a portion of the light traveling through the light guide. For example, see U.S. Pat. No. 5,339,382 issued Aug. 16, 1994 to Whitehead disclosing a prism light guide luminaire with a directional output. The prism light guide luminaire consists of a hollow structure made of transparent dielectric material with opaque and light emitting surface portions that together form a selected cross-sectional configuration that provides illumination along its length at a single circumferential location. The luminaire has either a circular or an elliptical cross-sectional shape. Another elongated light guide design is disclosed in U.S. Pat. No. 5,715,347 issued Feb. 3, 1998 to Whitehead. The disclosed apparatus utilizes a prism light guide with confocal opposed parabolic sections that is able to localize the transverse motion of light rays. This results in the seams at the intersections of the parabolas not affecting most light rays propagated along the guide.
The concept of longitudinally-extending light sources has extended to include waveguides. One such extension is described in U.S. Pat. No. 5,432,876 issued Jul. 11, 1995 to Appeldorn et al., which discloses optical fibers having notches of varying depth located along their length to produce lateral emission of the light traveling through the fibers. The notches are positioned at a particular angular location on the fibers so that lateral emission of light occurs at only a specific circumferential region.
U.S. Pat. No. 5,708,749 issued Jan. 13, 1998 to Kacheria discloses a light rod that emits light laterally out of a surface or end region as a result of using one or more surface treatments. The light rod can be solid or hollow and receives light from a remote source via optical fibers. The light rod cross-section is depicted as circular, but can be conical as well.
Waveguides have also been used in automotive lighting. One such use is disclosed in U.S. Pat. No. 5,495,400 issued Feb. 27, 1996 to Currie and disclosing an optical fiber illumination device for exterior vehicle lighting. The disclosed apparatus can utilize a round optical fiber side-light cable that extends about the periphery of a vehicle body component, such as about the periphery of a car window or truck bed.
Although the above-noted prior art includes various light source designs that provide circumferentially-limited lateral light emission, each involves some complexity in design or manufacturing that reduces its usefulness as a low-cost illumination source for automotive lighting. For the Whitehead designs, the apparatus is somewhat more complex, thereby increasing manufacturing costs. For the Appeldorn et al. design, the notches are created using a secondary operation that increases the cost of manufacturing.
It is therefore a general object of the invention to provide a low-cost illuminating waveguide design that provides circumferentially-limited lateral light emission along the length of the waveguide using a waveguide structure designed to efficiently direct the light laterally out of the waveguide at a particular circumferential location or region.